chapter 7. polymers in the liquid crystalline...

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Chapter 7. Polymers in the Liquid Crystalline State

7.1. Definition of a Liquid Crystal

7.2. Liquid Crystalline Mesophases

7.2.1. Mesophase topologies

7.2.2. Phase diagrams

7.2.3. First-order transitions

7.3. Classification of Liquid Crystal

7.3.1. Lyotropic liquid crystalline

7.3.2. Thermotropic liquid crystalline

7.3.3. Side-chain liquid crystalline

7.4. Thermodynamics and Phase Diagrams

7.4.1. Historical aspects

7.4.2. Importance of χ1 parameter

7.5. Fiber Formation

7.6 Comparison of Major Polymer Types

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Liquid Crystals

: substances that exhibit long-range order in one or two dimensions, but not all three.

: lie in-between of amorphous and crystalline in properties

: direct consequence of molecule asymmetry. It arises because two molecules

cannot occupy the same space at the same space time and is largely entropically

derived.

: For new classes of high-modulus fibers, high-temperature plastics and a host of

new electronic and data storage materials

Rod-Shape Chemical Structures

7.1. Definition of Liquid Crystal

NH NH

C

O

C

O

n

O C

O

O O C

O

C

O

n

Poly(p-phenylene terephthalamide) : Kevlar® by du Pont

Copolyester

HO2C CO2H HOOH

HO CO2H

Terephthalic acid Etylene glycol p-hydroxybenzoic acid

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Nematic : 1D organization of molecules

: with their chains lying parallel to each other at equilibrium

Smectic : ordered in 2D (exist a large number of smetic mesophases

Cholesteric : 2D twisted nematic mesophase

Discotic : like stacks of dishes or coins

Fig.7.1 Schematic representation of the

different types of mesophases: Smetic

with ordered (a) and unordered (a’); (b)

nematic; (c) cholesteric; and (d) discotic

7.2. Liquid Crystalline mesophases

7.2.1. Mesophase topologies

Some LC chemical structures

Fig 7.2

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Fig. 7.2 Asymmetric organic molecules in the form of rods or plates may form liquid crystal

structures

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7.2.2. Phase diagrams

Fig. 7.3 Phase diagrams illustrate the

expected behavior of a material as a

function of temperature and composition.

Here, a mixture of polymer

and the monomer

weight fraction

- filled symbols : DSC measurement

- open symbols : polarizing microscope

Note that the monomer by itself

crystallizes.

The nematic mixure clears in the

temperature range of 360 to 370 K

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※ LC - forming polymer may exhibit multiple mesophases at different Ts or Ps.

As T↑, the polymer goes through multiple first order transitions.

Fig. 7.4 Liquid crystal-forming

polymers may undergo many

first-order transitions. Here, as

the temperature is raised, the

polymer first melts to a smectic

structure, then to a nematic

structure, and then to an

isotropic melt.


7.2.3. First-order transitions

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Fig. 7.5 DSC heating trace of an acrylic, liquid

crystal-forming polymer.

Note two first-order transitions.

The nature of the first-order transitions is best illustrated by observing the

behavior of these materials in a differential scanning calorimeter (DSC).

CH2 CH

CO

O CH2 O C

O

O O CH2 CH

CH3

CH2 CH3

n

11

The presence of multiple endotherms

provides direct evidence of the

multiplicity of first-order transitions in this

polymer.

Tm=59℃ Smetic phase (via microscopy) Near 100℃ transform to a cholesteric

phase (however, the isotropic melt phase

appears at same temperature)

heating the polymer at a slower rate

separate the transitions

Not all polymers go through LC mesophases.

Polymers that form random coils melt directly to the isotropic liquid state.

Some portion of the chain or side chain must be rod- or disk-shaped to form a LC

mesophase.

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┌ Lyotropic - in concentrated (～30%) solution

│ Thermotropic - in melts

└ Mesogenic side group compositions - random coil backbone rod

shaped side group

(subclass of thermotropic LCs)


7.3.1. Lyotropic Liquid Crystalline Chemical Structures

Form nematic mesophases

Aromatic polyamides with aromatic ring structures, as shown in Table 7.1

Heterocyclic polymers yield materials with outstanding high-temperature

performance (see Table 7.2).have ladder or semi-ladder chemical structures

Some natural polymers include cellulose derivatives

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Table 7.1 Important polyamides yielding

liquid crystalline mesophase

aThe basis for the fiber “Kevlar® , widely used for bullet-proof jackets,

parachutes, etc.

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Table 7.2 Lyotropic solutions of polyheterocyclic compounds

Compound Structure Lyotropic Solvent Reference

Poly(1,4-phenylene-2,6-benzobisimidazole) Methanesulfonic acid (a)

Poly(1,4-phenylene-2,6-benzobisoxazole)

(PBO)

Methanesulfonic acid

Chlorosulfonic acid

100% sulfonic acid

(b,c)

(d)

Poly(1,4-phenylene-2,6-benzobisthiazole)

(PBT)

5-10% in polyphosphoric acid

Methanesulfonic acid

(e)

(c,d,f)

Poly[2,6-(1,4-phenylene)-4-phenylquinoline]1.0-1.5% in m-cresol-di-m-cresyl

phosphate(g)

Poly[1,1’-(4,4’-biphenylene)-6,6’-bis(4-

phenylquinoline)]

>9% in m-cresol-di-m-cresyl

phosphate(g)

Poly[2,5(6)-benzimidazole] (AB-PBI) Methanesulfonic acid (h)


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7.3.2. Thermotropic Liquid Crystalline

Aromatic copolyesters

Some homopolymer aromatic polyesters too high a melting temperature to form

thermotropic mesophases without decomposition

copolymerization reduces melting temperatures

Several techniques to reduce the melting temperature

① Copolymerization of several mesogenic monomers, which produces random copolymers with depressed melting temperature.

② Use of monomers with bulky side group, which prevents close packing in the LC mesophase, sometimes referred to as "frustrated chain packing".

③ Use of best comonomers to interrupt the order in the system④ Incorporation of flexible spacers, to decrease rigidity.

( └→Tg↓, solubility↑ )

This permits the development of bends or elbows in the chain.

Both Lyo- and Thermo- LCs are highly crystalline in actual usage, going from the LC

to the crystalline state by either removing the solvent or cooling the system

However, the side-chain mesogenic materials usually remain in the LC state for

their intended use.

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Table 7.3 Melting point versus structure for selected thermotropic LC polyesters

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Fig. 7.6 One way of classifying

thermotropic LC polymers is to examine

whether the mesogenic unit is in the

main chain or in the side chain

Flexible spacers placed in the main chain achieve both improved solubility and the

lowering of transition temperatures.

The flexible spacer may be hydrocarbon based, as in Table 7.3

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7.3.3. Side-chain Liquid Crystalline

Fig. 7.7 A schematic showing how

mesogenic LC side-chain polymers

can be organized into different

mesophases.

Polymers here have rod- or disk-shaped side groups placed on ordinary random

coil polymers, frequently acrylics or siloxanes (see Table 7.4)

The first attempts to form side-chain LC structures involved attachment of short

rod-shaped moieties directly to the main chain (Fig 7.7) disappointing

the reasons why

① Tg of polymers is always at much higher temperatures that for the corresponding polymers without mesogenic side chains

② During the polymerization process, the LC packing of the monomers was destroyed by steric requirements.

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Table 7.4 Examples of smectic polymers by lengthening the segments A and B

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Fig. 7.8 Oriented smectic LC polymers yield very distinctive X-ray patterns.

Here, the smectic mesophase (SA) is shown for polymers having the structures.

The structure of these LC phases was determined by X-ray analysis. (Fig 7.8)

CH2 C(CH3)

OC O (CH2)11 O CH N CN

CH2 C(CH3)

OC O (CH2)10 C CH N CNO

O

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Fig. 7.9 LC side chains can be packed in several arrangements, even considering only the

smectic A mesophase: (a) single-layer packing, (b) two-layer packing, (c) packing with

overlapping alkyl “tails,” and (d) packing with partial overlapping of the mesogenic side chains.

1, Main chain; 2, spacer; 3, mesogenic group; d, repeat distance, as revealed by X rays.


The packing arrangement of these materials is illustrated in Fig 7.9

The side chains can be arranged in either single-layer or double-layer packing

arrangements, Fig 7.9a and b, respectively.

Packing with partial overlap is also possible; Fig 7.9c and d

The selection of the probable packing mode was based on the d spacing shown in

Fig 7.9, which corresponds to the thickness of the smetic layer.

In contrast to the backbone types of LC, the side-chain types exhibit neither high

modulus nor high strength. However, provides highly interesting for structual and

optical properties (especially as transformed by electric and magnetic fields)

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LCs were first noted for organic molecules about 100 years ago; a peculiar melting

behavior of a number of cholesterol esters by Reinitzer. The crystals of the

substances melted sharply to form an opaque melt instead of the usual clear melt.

Lehmann reported the turbid states between the truly crystalline and the truly

isotropic fluid state introduced the term “Flussige Kristalle”, or liquid crystals

The first polymeric LC : poly(γ-benzly L-glutamate) in 1950


7.4.1. Historical Aspects

Flory. critical volume concentration

where : no. of isodiametric segments

→ axial ratio of the molecule

: a transition from complete disorder to partial order was predicted to occur

abruptly and discontinuously.

for high ,

*

2

8 21v

x x

8*

2

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(-) value does not affect the biphasic gap.

(+) value, critical volume exists at =0.055

=0.070 : triple point

1

1

1


7.4.2. Importance of χ1 parameter

The statistical thermodynamic theory described above pertains to rods devoid of

interactions other than the short-range repulsions, which preclude intrusion of one

rod on the space occupied by another.

theoretical deductions stem solely from the geometrical aspects of the molecules

Introduction of the polymer-solvent interaction parameter χ1, for lyotropic systems

Fig 7.10

Fig. 7.10 Phase diagram for rods of axial ratio x=100.

The positions of the coexisting phases are

determined by the value of the parameter χ. The

minimum of the shallow concave portion of the

diagram is a critical point marking the emergence of

two anisotropic phases, in addition to the isotropic

phase appearing on the left of the figure.

dilute isotropic

dilute anisotropic

concentrated anisotropic

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① Optical pattern or texture observations with a polarizing microscope.

┌ Isotropic liquid - no texture

└ liquid crystals - have the texture

② Nematic and smectic phases can be distinguished by DSC on the basis of the magnitude of the enthalpy changes accompanying the transition to the

isotropic phase. → measure △Htrans③ Miscibility with known liquid crystals to form isomorphous mesophases.

See Fig 7.3

④ Possibilities of inducing significant molecular orientation by either supporting

surface treatment or external electrical or magnetic fields.

⑤ Small-angle X-ray to study molecular long-range order

⑥ Small-angle light scattering between crossed nicols

different patterns for different mesophases.

※ Mesophase Identification in Thermotropic polymers

A major question in the LC field the identification of the various mesophases

Several methods may be used

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S=± ½ or ±1 Nematic (indicated by a mixture of two and four point disclinations)

S=±1 Smectic (exhibits only four point diaclinations)

※ Mesophase Identification in Thermotropic polymers

The most widely used method : optical microscopy (between crossed polarizers)

Identify the appearance of mesophases and transitions between the various

mesophases and isotropic materials

Many LC polymers exhibit Schlieren textures : display dark brushes See Fig 7.11

correspond to extinction positions of the mesophase

At certain points, two or more dark brushes meet : disclinations (like dislocations in c

rystalline solid, where domains of differing orientation melt)

The disclination strength is calculated from the number of dark brushes meeting at

one point:

4

number of brushes meetingS

S : positive when the brushes turn in the same direction as the rotated polarizers

: negative when they turn in the opposite direction

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Fig.7.11 Optical microscopy reveals a Schlieren texture for a copolyester formed by the

transesterification of poly(ethylene-1,2-diphenoxyethane-p,p'-dicarboxylate) with p-

acetoxybenzoic acid. Recognizable singularities S=±1/2 are marked with arrows. Crossed polarizers, 260℃, ×200

Mesophase Identification in Thermotropic polymers

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The viscosity of lyotropic solutions in shear

flow is lower than that of random coil solutions

of the same molecular weight and

concentration.

⇒ due to the lack of molecular entanglementsin the nematic phase.

7.5. Fiber Formation

Fig. 7.12 The viscosity of rod-shaped polymers usually

goes through a maximum as the chains organize from

the isotropic state to a mesostate. Data for poly(p-

phenylene benzobisoxazole) in concentrated sulfuric

acid.

Viscosity of Lyotropic Solutions

Molecular Orientation

through spinning, shear induced orientation

achieved on removing the solvent, polymer

crystallizes because of the lack of chain

foldings and other perfections, the fibers

have higher moduli and higher strength.

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7.6. Comparison of Major Polymer Types

Table 7.5 Comparison of major polymer types

Property Rigid Rod Extended Chain Random Coils

LC v2* critical concentration, % 4-5 14-15 None

Dilute solution

conformation

TgTf

Rod

Noa

Nob

Worm

Noa

Nob

Coil

Yes

Yes

Persistence length, Å >500 90-130 10

Mark-Houwink a value 1.8 1.0 0.5-0.8

Catenation angle, degrees 180 150-162 109-120

Max [η], dl/g 48-60 15-25 0.1-3

aMay be difficult to observe if highly crystalline.bMay decompose before melting.

Molecular conformation

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7.6. Comparison of Major Polymer Types

Fig. 7.13 Rigid-chain and semiflexible-chain ordered assemblies as the concentration of the

polymer is increased.

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Regardless of molecular size or shape, a liquid crystal must satisfy three basic

requirements.

1. There must be a first-order transition between the true crystalline state at the

lower T found leading to the liquid crystalline state, and another first-order

transition leading to the isotropic liquid state (or another liquid crystal state) at

the upper T found of the liquid crystalline state.

2. A liquid crystal must exhibit one-or-two dimensional order only : true crystals

have 3D order, and the isotropic liquid is completely disordered.

3. A liquid crystalline material must display some degree of fluidity

: although for polymers the viscosity may be high.

※ Basic Requirements for Liquid Crystal Formation

Experiment Amorphous Crystalline Liquid Crystalline

X-ray Amorphous halo 3-D order 1- or 2-D order

Polarizing microscope No texture Spherulites Schlieren texture

DSC, dilatometric Only Tg Tg and Tf Tg and two or more

first-order transitions

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Keywords in Chapter 8

- Hydrophilic-lipophilic balance (HLB)

- Critical packing parameter

- χN vs f phase diagram for block copolymers

- Phase diagram for ternary amphiphilic block

copolymer – water – oil system

- Applications of copolymers

chapter 7. polymers in the liquid crystalline...

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